The present invention relates generally to four wire leased line telecommunication circuits, and more particularly to improved methods of troubleshooting for such circuits where V.32 technology modems or other unbalanced modems are utilized.
V.32 modems are desirable because they provide the ability to use a PSTN (Public Switched Telephone Network) to back up a leased line facility with a single two wire dial telephone line. The dial facilities employ significantly different routing from the leased line facilities, and therefore provide reliable service when leased facilities are inoperative.
In addition, V.32 modems used in the four wire leased line mode are relatively low priced compared to modems which use prior technologies, such as V.29 and V.27. However, the V.32 modem possesses certain disadvantages relative to V.29 or V.27 modems when the leased line facility or local equipment is down (i.e., has experienced a failure). Techniques previously established for testing the leased line facility have utilized a loopback provided by the facility telephone company, but this CCITT V.45 loop 4 which is established "in a separate unit" (that is, not in the Data Communication Equipment, or DCE) is not usable with conventional V.32 modems.
The problem with using current loopback testing techniques is that the V.32 modem specification is based on a dial environment and is not symmetrical (i.e., is unbalanced). The modem specification defines an originate entity and an answer entity which are non-symmetrical because their functions and operation are different and are not interchangeable. An example of the differences is that the answer modem initially transmits tones to begin the "handshake" sequence while the originate modem is silent. That is, in a standard call set up procedure, the originate side of the connection sought to be established must receive an answer back tone at its receiver before its transmitter is capable of sending information. There are other differences in the performance of the two modems both during handshake and afterward, but this is the principal difference which creates the need for differences in the testing procedure at installation and for maintenance, when compared to prior modem technologies. For example, V.29 and V.28 modem leased line technologies are balanced in that, while both ends of a circuit have independent transmit and receive functions, there is no apparent difference in operation between the two ends, at least for testing purposes.
It is therefore a principal object of the present invention to provide improved methods an- systems to allow troubleshooting four wire leased line telecommunication circuits when V.32 technology modems are employed in conjunction with such circuits.